1. Field of the Invention
The invention is in the general field of communications protocols and methods, and more specifically in methods of modulating communication signals that are resistant to echo reflections, frequency offsets, and other communications channel impairments.
2. Description of the Related Art
Modern electronics communications, such as optical fiber communications, electronic wire or cable based communications, and wireless communications all operate by modulating signals and sending these signals over their respective optical fiber, wire/cable, or wireless mediums. These signals, which generally travel at or near the speed of light, can be subjected to various types of degradation or channel impairments. For example, echo signals can potentially be generated by optical fiber or wire/cable medium whenever the modulated signal encounters junctions in the optical fiber or wire/cable. Echo signals can also potentially be generated when wireless signals bounce off of wireless reflecting surfaces, such as the sides of buildings, and other structures. Similarly frequency shifts can occur when the optical fiber or wire/cable pass through different regions of fiber or cable with somewhat different signal propagating properties or different ambient temperatures; for wireless signals, signals transmitted to or from a moving vehicle can encounter Doppler effects that also result in frequency shifts. Additionally, the underlying equipment (i.e. transmitters and receivers) themselves do not always operate perfectly, and can produce frequency shifts as well.
These echo effects and frequency shifts are unwanted, and if such shifts become too large, can result in lower rates of signal transmission, as well as higher error rates. Thus methods to reduce such echo effects and frequency shifts are of high utility in the communications field.
In parent application Ser. No. 13/117,119, a novel method of wireless signal modulation was proposed operated by spreading data symbols over a larger range of times, frequencies, and spectral shapes (waveforms) than was previously employed by prior art methods (e.g. greater than such methods as Time Division Multiple Access (TDMA), Global System for Mobile Communications (GSM), Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency-Division Multiplexing (OFDM), or other methods). This newer method, which in Ser. No. 13/117,119 was termed “Orthonormal Time-Frequency Shifting and Spectral Shaping (OTFSSS)”, and which here will be referred to by the simpler “OTFS” abbreviation, operated by sending data in larger “chunks” or frames than previous methods. That is, while a prior art CDMA or OFDM method might encode and send units or frames of “N” symbols over a communications link over a set interval of time, the Ser. No. 13/117,119 invention would typically be based on a minimum unit or frame of N2 symbols, and often transmit these N2 symbols over longer periods of time. With OTFS modulation, each data symbol or element that is transmitted is spread out to a much greater extent in time, frequency, and spectral shape space than was the case for prior art methods. As a result, at the receiver end, it will generally would take longer to start to resolve the value of any given data symbol because this symbol must be gradually built-up or accumulated as the full frame of N2 symbols are received.
Put alternatively, parent application Ser. No. 13/117,119 taught a wireless combination time, frequency and spectral shaping communications method that transmitted data in convolution unit matrices (data frames) of N×N (N2), where generally either all N2 data symbols are received over N spreading time intervals (each composed of N time slices), or none are. To determine the times, waveforms, and data symbol distribution for the transmission process, the N2 sized data frame matrix would be multiplied by a first N×N time-frequency shifting matrix, permuted, and then multiplied by a second N×N spectral shaping matrix, thereby mixing each data symbol across the entire resulting N×N matrix (termed the TFSSS data matrix in '119). Columns from this N2 TFSSS data matrix were then selected, modulated, and transmitted, on a one element per time slice basis. At the receiver, the replica TFSSS matrix was reconstructed and deconvoluted, revealing the data.